I mentioned Kerio Archiver for user mailboxes the other day and had several customers ask me what effect archiving messages might have on performance. That's a hard question to answer, because performance can be affected by so many things, but there is a way to get an idea.

Having a large number of items in a folder will mean than operations in that
folder will take longer. Operations that depend on the number of items in
the folder include adding a new column to the view, sorting on a new column,
finds and searches. Many Outlook plug-ins do sorts or searches as they are
running, and these requests may overlap with other Outlook MAPI requests,
resulting in a poor user experience.

What are key folders? For Kerio, those would be INBOX and Sent Items. For users who refuse to clean Deleted Items, we can add that. The INBOX is of course the most heavily used - every time new email arrives that folder must be updated and every time a user accesses their email it has to be searched and sorted.

You can get an idea of how much of that goes on by turning on Message Store -> Searching and Sorting in your debug log. First, right click in the log itself and select "Messages".

Then choose "Searching and Sorting" under Message Store.

You'll see messages like this as people log in and out:

That's from my session with only a few hundred messages in my INBOX.
Don't forget to turn that debug message off when you have what you need.

Looking at that for your large mailboxes should give you an idea of the workload caused by these. However, I do need to mention that archiving is not the whole solution when mailboxes have been allowed to grow unchecked. The problem is that when large folders are created, deleting messages leaves just as many empty slots which do have to be searched - you'll save on sorting time, but searching will take almost as long.

There is a way to fix that: stop the Kerio server and (using your operating system tools) rename the INBOX to another name. Restart the server and when the user accesses mail, an empty INBOX will be created for them. If you regularly use a tool like the Archiver after that, the mailbox will not have that "empty slot" issue.